1. Field of the Invention
This invention relates to the reduction of reflection loss from transparent optics of IR optical systems or instruments and more particularly, to anti-reflection coatings which are particularly effective when applied to mixed crystals of thallium iodide and thallium bromide in order to give transparent optics, such as windows, lenses and optical fibers, of IR optical systems.
2. Description of the Prior Art
A mixed crystal of thallium iodide (TlI) and thallium bromide (TlBr) which is usually called KRS-5, and hereinafter referred to as such, has a refractive index of as high as 2.37 at a wavelength of 10.6 microns. This results in a reflection loss of about 28% at both end surfaces of transparent optics made of the mixed crystal. In order to reduce the loss, it is desirable to coat an anti-reflection film at each end surface of the optical part. However, no technique of providing an anti-reflection coating on the KRS-5 material has been established yet.
In recent years, the KRS-5 material has been reduced to practice as an optical fiber for carbon dioxide laser scalpels. There is a high demand of an anti-reflection coating or film for improvement of the optical transmission.
The anti-reflection film which is to be applied to the optical fiber at opposite end surfaces thereof is required to have a much higher laser irradiation damage threshold level than that of an ordinary window or lens for IR rays because the carbon dioxide laser beam is used. With the optical fiber, a laser beam is ordinarily focussed and passed to the optical fiber through one end surface thereof, so that a power density on the end surface becomes greater by about two digits than that of ordinary laser optics. For instance, when a laser beam system is operated at an output power of 50 W, a power density at the incident surface of an optical fiber reaches as high as 50 W/cm.sup.2. Accordingly, the anti-reflection coating on each side of the optical fiber is required to have not only high water-proof or moisture-proof properties and good adherence, but also an extremely high laser irradiation damage threshold.
In order to improve the threshold level, it is necessary to provide an anti-reflection film having a laser absorptance which is as small as possible.
Optically transparent optics for carbon dioxide laser are conventionally made of zinc selenide (ZnSe), gallium arsenide (GaAs), potassium chloride (KCl) and the like. Anti-reflection coatings or films for these materials have been proposed including those of single-layer, double-layer and three-layer structures. Anti-reflection films having a multi-layer structure having four or more superposed layers therein may be formed. However, such a multi-layer structure has disadvantages in that formation of the multi-layer structure by vacuum deposition becomes more complicated and that absorption of the laser beam increases with an increase of the layer thickness. Accordingly, the anti-reflection film is usually limited to those consisting of three layers at most.
An anti-reflection coating of a single-layer structure can be most easily formed. According to the optical theory, when a film material whose refractive index, n, is equal to a square root of a refractive index, .sqroot.n.sub.s, of a material of substrate is vacuum deposited on a substrate in an optical thickness, nd, which satisfies nd=.lambda./4, e.g. when .lambda.=10.6 microns, nd=2.65 microns, the reflectance becomes zero. Thus, an ideal anit-reflection film of a single layer is obtained. Only a limited number of materials are known which satisfy the above requirement. Materials which have a refractive index, n, near the square root of a refractive index, .sqroot.n.sub.s, of the KRS-5 material, i.e. .sqroot.n.sub.s =.sqroot.2.37=1.54, are indicated in Table 1 below along with reflectance, absorptance, and waterproof characteristics.
TABLE 1 ______________________________________ Reflectance Absorptance per Film per Film n (10.6 microns) (calculated) (calculated) Water Proof ______________________________________ BaF.sub.2 1.39 1% 0.3% poor ThF.sub.4 1.41 0.8% 0.6% good PbF.sub.2 1.67 0.7% 0.05 poor ______________________________________
As will be seen from the above table, the reflectance cannot be zero in the case of these single-layer anti-reflection films, which have a reflectance of about 1%. With ThF.sub.4, its waterproof is excellent but it has the drawback of the high absorptance. PbF.sub.2 has a small absorptance but is poor in waterproof. That is, the vacuum-deposited PbF.sub.2 film is not resistant to water. When the layer contacts with water on the surface thereof by accident, it is readily cracked with an attendant increase of light scattering.
In the case of the double-layer structure, a combination of two dielectric materials which satisfy Schuster's equation concerning the double-layer structure will theoretically permit the reflectance to be zero. Examples of such a combination are indicated in Table 2 below.
TABLE 2 ______________________________________ Reflectance per Film Absorptance (calculated) per Film Water Proof ______________________________________ BaF.sub.2 /ZnSe 0% 0.2% moderate ThF.sub.4 /ZnS 0% 0.4% good ZnS/Ge 0% 0.19% moderate ______________________________________
As will be seen from the above table, the double-layer films have theoretically a reflectance of zero but have disadvantageously large absorptances.